1. Field of the Invention
The invention relates to a transmission liquid crystal display with a reduced dependency of a display quality upon a visual angle.
2. Description of the Related Art
FIG. 1 is a fragmentary cross sectional elevation view illustrative of a conventional transmission liquid crystal display. The conventional transmission liquid crystal display has a surface light source 201 and a liquid crystal panel 211. The surface light source 201 may comprise either a cold cathode fluorescent lamp optoguiding plate. The liquid crystal panel 211 includes a deflection plate 202, an active matrix substrate 203 laminated on the deflection plate 202, a color filter substrate 205 provided to be spaced apart from the active matrix substrate 203 through sealing resin agents 206 to define, between both the active matrix and color filter substrates 203 and 205, a space filled with a liquid crystal 204 with a twist angle of 90 degrees. The active matrix substrate comprises a transparent glass substrate on which a number of sets of thin film transistors and transparent pixel electrodes are aligned in matrix. The color filter substrate 205 comprises a transparent common electrode and a color filter. The sealing resin agents 206 provides both functions of an adhesion between the active matrix and color filter substrates 203 and 205 and a sealing for the liquid crystal between those substrates 203 and 205. There is further provided on the color filter substrate 205 a deflection plate 207 with a deflection axis deflected by 90 degrees, in the normally white mode, from a deflection axis of the deflection plate 202.
In the above liquid crystal display, the common electrode on the color filter substrate 205 is applied with a predetermined fixed voltage, while the transparent pixel electrodes of the active matrix substrate 203 are applied with variable voltages according to image data to be displayed. As a result, a magnitude of the twist of the liquid crystal 204 between the above two substrates 203 and 205 is changed thereby a brightness of the display light emitting from the pixel electrodes is modulated.
The liquid crystal employed in such liquid crystal display possesses a refraction index anisotropy which causes, in a large visual angle, an inversion of gray scales and a reduction in a ratio of contrast, resulting in a considerable deterioration of display quality. In the above liquid crystal display, concretely, the visual angle which, beyonds 10 degrees downward, causes the gray scale inversion and the visual angles which, beyond 20 degrees upward and 50 degrees downward, cause a ratio of the contrast to be reduced down to 10 or less.
The above transmission liquid crystal display illustrated in FIG. 1 has a disadvantage in a narrow visual angle. A large deflection from the normal line of the liquid crystal panel results in a large difference in a retardation of the transmitted lights thereby the graduation property or the gray scale property is deteriorated. This results in a lowering of the ratio of contrast and thus resulting in a difficulty in obtaining a wide visual angle image.
To settle the above problems, there have been proposed some techniques to reduce the dependency of the display quality upon the visual angle of the transmission liquid crystal display. FIG. 2 is a cross sectional elevation view illustrative of another conventional transmission liquid crystal display with an improved dependency of the display quality upon the visual angle. The conventional transmission liquid crystal display has a surface light source not illustrated, a liquid crystal panel 311, a collimator 314 and a plurality of micro-lenses 308. The surface light source may comprise either a cold cathode fluorescent lamp or an optoguiding plate. The liquid crystal panel 311 includes a deflection plate 302, a scanning electrode substrate 203 laminated on the deflection plate 302, a scanning electrode 303a provided on the scanning electrode substrate 303, a signal electrode substrate 305 with a plurality of signal electrodes, which is provided to be spaced apart from the signal electrode substrate 303 through sealing resin agents 306 to define, between both the scanning electrode and signal electrode substrates 303 and 305, a space filled with a liquid crystal 304. The sealing resin agents 306 provides both functions of an adhesion between the scanning electrode and signal electrode substrates 303 and 305 and a sealing for the liquid crystal between those substrates 303 and 305. There is further provided on the signal electrode substrate 305 a deflection plate 207. On the deflection plate 307, there is provided the collimator 314 which allows a transmission only of a vertical-to-display surface component of the light. For every pixel of the display panel, the micro-lenses 308 are provided on the collimator 314.
The provision of the collimator 314 permits the transmission of the vertical-to-surface component of the light transmitted through the liquid crystal panel 211. If no micro-lens were provided, the vertical-to-surface component results in the best display property but a too narrow visible angle to perform the display die to a strong directivity. Accordingly, the above display is provided with the micro-lenses 308 in front of the collimator 314 to cause an optical diffusion for widening the visual angle.
The above display faces to a problem with a difficulty in a high accuracy alignment of the micro-lenses on the corresponding pixels of the liquid crystal panel. A further disadvantage of the above display is a difference in thermal expansion between a glass material for the glass substrate constituting the liquid crystal panel 311 and the micro-lenses 308 causes a variation of temperature to generate a dislocation between the liquid crystal panel 311 and the micro-lenses 308. The dislocation between them causes a deterioration in quality of the display. Further, the micro-lenses are positioned at a direct viewing position. The micro-lenses are transparent to the light, but have no mirror face, and thereby a diffused reflection is caused to look whitish in color. Further there is caused a reflection of the external light thereby resulting in a deterioration in a visual sense of light.
To settle the above problem, it has been proposed to provide still another transmission liquid crystal display with more fine micro-lenses, the number of which is larger than the number of the pixels. FIG. 3 is a cross sectional elevation view illustrative of this transmission liquid crystal display device. The conventional transmission liquid crystal display has a surface light source 401 and a liquid crystal panel 411. The surface light source 401 may comprise either a cold cathode fluorescent lamp or an optoguiding plate. The liquid crystal panel 411 includes a deflection plate 402, an active matrix substrate 403 laminated on the deflection plate 402, a color filter substrate 405 be spaced apart from the active matrix substrate 403 through sealing resin agents 406 to define, between both the active matrix and color filter substrates 403 and 405, a space filled with a liquid crystal 404 with a twist angle of 90 degrees. The active matrix substrate comprises a transparent glass substrate on which a number of sets of thin film transistors and transparent pixel electrodes are aligned in matrix. The color filter substrate 405 comprises a transparent common electrode and a color filter. The sealing resin agents 406 provides both functions of an adhesion between the active matrix and color filter substrates 403 and 405 and a sealing for the liquid crystal between those substrates 403 and 405. There is further provided on the color filter substrate 405 a deflection plate 407 with a deflection axis deflected by 90 degrees, in the normally white mode, from a deflection axis of the deflection plate 402. Moreover there is provided an optical diffusion lens in front of the liquid crystal panel 411.
In the above transmission liquid crystal display, light is emitted from the surface light source 401 and then transmitted through the liquid crystal panel 411 into the optical diffusion lens 408. A front light showing the best graduation or gray scale is diffused to have upward and downward visual angles, while upward and downward regions visual fields of the display region generate a light with a broken graduation or the gray scale which is then also diffused by the optical diffusion lens 408, resulting in a relaxation of the graduation inversion or the gray scale inversion and accordingly the visual angle is widened.
The above transmission liquid crystal display of FIG. 3 has the following disadvantages. The optical diffusion lens is positioned at a direct viewing position. Although the optical diffusion lens is transparent to the light but have no mirror face thereby a diffused reflection is caused to look into a whitish color. Further there is caused a reflection of the external light thereby resulting in a deterioration in a visual sense of light. The distance between the liquid crystal panel and the optical diffusion lens is large thereby causing a display blur. Namely, the display blur is caused due to the following two factors. The first factor is a scattering at a lens surface and a reflection of the light for a focus of the light. The second factor is a spread of the emission light at a predetermined angle. The width of the light beam with the spread is increased as the above distance becomes large. FIG. 4 is illustrative of the light beam spread. The light having a spread angle emits from the color filter 405 and then be transmitted with showing a spread through the transparent glass substrate 405a and the deflection plate 407. Although the light with a width "a" at the color filter 405 should ideally be observed, the light is actually spread to a width "a'" together with a scattering and a refraction just before observation. This is the cause of the display blur. The magnitude of the spread or the blur is proportional to a distance between the color filter 405 and the optical diffusion lens 408.
There has been yet another proposal for a combination of the liquid crystal panel with optical elements such as optical lenses for improvement in the display quality. The Japanese Utility Model Application No. 62-147947 discloses a transmission liquid crystal display provided with a linear Fresnel lens for optical condense within the visual angle in front of a liquid crystal panel. The Japanese Laid-open Patent Application No. 60-262131 discloses another transmission liquid crystal display wherein lenticular lenses or compound eyes-lenses are provided at both a light source side and a display side of the liquid crystal panel so as to correspond 14 focus both the lenticular and the compound eyes lenses at the liquid crystal side.
There had been proposed in the prior art no ideal transmission liquid crystal display free from any problems and any disadvantages as described above. It has therefore been required to develop a quite novel transmission liquid crystal display free from any problems and any disadvantages as described above.